Is she seeing someone?

Just wondering what people think about the following situation:

There's a girl at work I really like but the problem I think she may be seeing someone else. I meet up with her at lunch fairly frequently and I have tried to let her know I am interested by increased eye contact, jokey texts/emails.

Suprisingly, she actually asked me out recently, for a night out with some of her friends. I went, though am pretty sure one of the guys she invited out she is seeing. She is generally quite a touchy person (hitting people playfuly a lot etc) but her whole body language seemed to be geared towards him. At one point she rested her head on his shoulder but there was nothing too obvious like kissing or holding hands.

I'm just wondering whether people think a girl would invite a guy out to show him that she is seeing someone? I think she realises that I like her but surely a better way would be for her to casually mention she is seeing someone?

I'm a bit confused as it was not totally obvious that she was with this other guy, it just seemed like there was something going on there.